In the field of audio electronics, it is often desireable to provide a user-controlled volume feature in an audio component, such as a compact disc player or the like. It is, therefore, common for such components to include gain or attenuation control circuitry for providing such volume control.
With the recent advancement in digital signal processing technology, most audio components found today also include digital circuitry having signal conversion elements such as digital-to-analog converters [hereinafter "DACs"] and analog-to-digital converters [hereinafter "ADCs"]. Such elements, like those of many electrical circuits have DC offset error voltages associated therewith. These offset error voltages can introduce undesired effects in the output signal. Thus, many systems include calibration circuitry for use in minimizing the undesired effects of the offset error voltages. Particularly, DACs and ADCs are commonly calibrated such that any significant offset error voltage is effectively nullified and a desired output signal results from a known input signal. One example of such a calibration scheme is taught in U.S. Pat. No. 5,087,914 to Sooch et al. In the Sooch' 914 patent, a digital correction value for the DAC offset error voltage is determined during a calibration routine and is subtracted from each digital input to the DAC during normal operation. Thus, for a predetermined input of, for example midscale code, a known output signal of, for example, zero volts results. Other similar calibration schemes achieve this same result.
In certain digital audio applications, such calibration schemes are effective at minimizing the deleterious effects of offset voltages. There exist, however, digital audio applications where these calibration schemes produce unwanted results. Specifically, in applications where a constant average DC output voltage is desired for a known input signal, regardless of the attenuation setting, such calibration schemes fail. An example of such an application includes an audio component with digital conversion circuitry (i.e., a DAC) as well as a gain control circuit (providing control of gain and attenuation), and the output of the DAC is AC coupled by a user to external subsequent components such as an amplifier and/or speaker system. With such a circuit arrangement, for a known input of midscale code, a constant average DC output signal (of any value) is desired, regardless of the level of attenuation provided by the gain control circuit. This is so because any discrete changes in the average DC value of the output signal, caused from a discrete change in the level of attenuation (i.e., digital volume control adjustment), will be AC coupled to the speaker system and be heard as a "click" or "pop" by the user. The prior art calibration schemes, described above, produce this unwanted result.